As fruit grow, proportions of cell wall, carbohydrate, organic acid, lipid, phospholipid and volatile (aroma) compounds change dramatically; and within each of those groups there are changes in the proportion of individual group members. Of these, by far the most important in practical terms is carbohydrate economy. Two sets of issues are at stake: (1) rate of growth, attainment of maturity and final fruit size, and (2) aroma, flavour and texture in ripe fruit. Both carry commercial implications.
Enlarging fruit require carbohydrate to sustain cell division, enlargement and tissue specialisation. Only in later stages are carbohydrates typically retained as either starch or soluble sugars. Soluble carbohydrate is mainly imported as photoassimilate, with only a minor contribution from local CO2 fixation, and reassimilation of respiratory CO2.
During peak fruit expansion, usually early summer, there is an intense flow of photoassimilate from mature leaves (sources) into rapidly enlarging fruit (sinks). Sugars generated by photosynthesis, along with amino acids and phosphate within the plant’s vascular network, move via the phloem into enlarging fruit.